This application claims priority to French patent application number 01 03 106 filed Mar. 7, 2001.
The present invention concerns production of acrylic acid from propane in the absence of molecular oxygen.
It is known from European Patent Application No. EP-A-608838 to prepare an unsaturated carboxylic acid from an alkane according to a vapour-phase catalytic oxidation reaction in the presence of a catalyst containing a mixed metal oxide comprising, as essential components, Mo, V, Te, O as well as at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium, these elements being present in well-specified proportions. The uses of such a silicon-free catalyst, which are described in the examples of this document, lead to good selectivities for acrylic acid, but they are implemented in the presence of air.
The European Patent Application No. EP-A-895809 describes catalysts based on oxides comprising molybdenum, vanadium, niobium, oxygen, tellurium and/or antimony, as well as at least one other element such as iron or aluminium. These catalysts can be used for the conversion of propane into acrylic acid, but only conversion in the presence of molecular oxygen is envisaged. They may also comprise a support such as silica, although the only examples of this document relating to the production of acrylic acid, namely Examples 9 and 10, implement silica-free catalysts.
The European Patent Application No. EP-A-603836 relates to a process for preparing a catalyst for the production of a nitrile. This catalyst may be a complex oxide comprising molybdenum, vanadium, tellurium, oxygen as well as at least one other element, which may bexe2x80x94among other elementsxe2x80x94niobium or antimony. The examples of this patent application describe preparations of catalysts which furthermore contain silica.
The Japanese Patent No. JP 4235153 concerns the production of acrylonitrile from propane and ammonia, according to a redox process.
The Applicant has now discovered that acrylic acid can be manufactured by gas-phase oxidation of propane in the absence of molecular oxygen, by passing a gaseous mixture of propane and steam and, optionally, an inert gas, over a particular solid composition of mixed oxides, which acts as a redox system and provides the oxygen necessary for the reaction.
The advantages of this novel process are as follows:
the limitation of the over-oxidation of the products which are formed, which takes place in the presence of molecular oxygen; according to the present invention, because the operation is carried out in the absence of molecular oxygen, the formation of COx (carbon monoxide and carbon dioxide), which are breakdown products, is reduced, which makes it possible to improve the selectivity for acrylic acid;
the selectivity for acrylic acid remains good when the reduction factor of the solid composition increases;
the solid composition, once it has undergone reduction and a progressive loss of its activity, can be regenerated easily by heating in the presence of oxygen, or a gas containing oxygen, after a certain period of use; after regeneration, the solid recovers its initial activity and can be used in a new reaction cycle;
the separation of the steps of reducing the solid composition and regenerating it makes it possible:
to increase the selectivity for acrylic acid;
to increase the partial pressure of propane, such a partial pressure of propane in the feed being no longer limited by the existence of an explosive zone created by the propane+oxygen mixture.
The present invention therefore relates to a process for manufacturing acrylic acid from propane, characterized in that a gas mixture, which is free from molecular oxygen and comprises propane, steam as well as, optionally, an inert gas, is passed over a solid composition of formula (I)
Mo1VaTebNbcSidOxxe2x80x83xe2x80x83(I)
in which:
a is between 0.006 and 1, including the end points;
b is between 0.006 and 1, including the end points;
c is between 0.006 and 1, including the end points;
d is between 0 and 3.5, including the end points; and
x is the quantity of oxygen bound to the other elements, and depends on their oxidation states,
in order to oxidize the propane according to the following redox reaction (1)
SOLIDoxidized+PROPANExe2x86x92SOLIDreduced+ACRYLIC ACIDxe2x80x83xe2x80x83(1).
This process makes it possible to obtain a selectivity of close to 60% for acrylic acid. Furthermore, if the propylene, which is a by-product, and the unconverted propane are recycled, an overall selectivity of close to 70% is achieved.
Other characteristics and advantages of the invention will become apparent on reading the following explanation, which is illustrated by examples.
The catalyst used according to the invention satisfies the formula (I) indicated above.
The oxides of the various metals involved in the composition of the mixed oxide of formula (I) can be used as starting materials in the preparation of this composition, although the starting materials are not limited to the oxides; the following may be mentioned as other starting materials:
in the case of molybdenum: ammonium molybdate, ammonium paramolybdate, ammonium heptamolybdate, molybdic acid, molybdenum halides or oxyhalides such as MoCl5, organometallic compounds of molybdenum, e.g. molybdenum alkoxides such as Mo(OC2H5)5, acetylacetone molybdenyl;
in the case of vanadium: ammonium metavanadate, vanadium halides or oxyhalides such as VCl4, VCl5 or VOCl3, organometallic compounds of vanadium, e.g. vanadium alkoxides such as VO(OC2H5)3;
in the case of tellurium: telluric acid;
in the case of niobium: niobic acid, Nb2(C2O4)5, niobium tartrate; niobium hydrogen oxalate, oxotrioxalatoammonium niobate [(NH4)3[NbO(C2O4)3]. 1.5H2O], ammonium niobium oxalate, ammonium niobium tartrate, niobium halides or oxyhalides such as NbCl3, NbCl5 and organometallic compounds of niobium, e.g. niobium alkoxides such as Nb(OC2H5)5, Nb(O-n-Bu)5;
and, in general, all the compounds capable of forming an oxide by calcination, namely the metal salts of organic acids, the metal salts of inorganic acids, metal complex compounds, etc.
The source of silicon generally consists of colloidal silica.
According to particular embodiments, the solid compositions of formula (I) may be prepared by mixing, while stirring, aqueous solutions of niobic acid, ammonium heptamolybdate, ammonium metavanadate and telluric acid, preferably while adding colloidal silica, then pre-calcining under air at about 300xc2x0 C. and calcining under nitrogen at about 600xc2x0 C.
Preferably, in the solid composition of formula (I):
a is between 0.09 and 0.8, including the end points;
b is between 0.04 and 0.6, including the end points;
c is between 0.01 and 0.4, including the end points; and
d is between 0.4 and 1.6, including the end points.
According to the invention, acrylic acid is manufactured by passing a gas mixture, which is free from molecular oxygen and comprises propane and steam as well as, optionally, an inert gas, over a solid composition of formula (I) as defined above, in order to carry out the redox reaction (1) as indicated above.
Generally, the redox reaction (1) is carried out at a temperature of 200 to 500xc2x0 C., preferably 250 to 450xc2x0 C., and even more preferably 350 to 400xc2x0 C.
The pressure is generally from 1.01xc3x97104 to 1.01xc3x97106 Pa (0.1 to 10 atmospheres), preferably from 5.05xc3x97104 to 5.05xc3x97105 Pa (0.5-5 atmospheres).
The residence time is generally from 0.01 to 90 seconds, preferably from 0.1 to 30 seconds.
The volume ratio of propane/steam in the gas phase is not critical, and may vary in wide limits.
Similarly, the proportion of inert gas, which may be helium, krypton, a mixture of these two gases, or alternatively nitrogen, carbon dioxide, etc., is not critical either, and may also vary in wide limits.
As an order of magnitude for the proportions of the initial mixture, the following ratio (by volume) may be mentioned:
propane/inert (Hexe2x80x94Kr)H2O (steam): 10-20/40-50/40-50
During the redox reaction (1), the solid composition undergoes reduction and a progressive loss of its activity. This is why, once the solid composition has been converted at least partially into the reduced state, regeneration of the said solid composition is carried out according to the reaction (2):
SOLIDreduced+O2xe2x86x92SOLIDoxidizedxe2x80x83xe2x80x83(2)
by heating in the presence of oxygen, or a gas containing oxygen, at a temperature of 250 to 500xc2x0 C. for the time needed to re-oxidize the solid composition.
Generally, the process is implemented until the reduction factor of the solid composition is between 10 and 40%.
This reduction factor may be monitored during the reaction via the quantity of products obtained. The equivalent quantity of oxygen is then calculated. It may also be followed via the exothermicity of the reaction.
After the regeneration, which may be carried out under temperature and pressure conditions identical to, or different from, those of the redox reaction, the solid composition recovers an initial activity and can be used in a new reaction cycle.
The redox reaction (1) and the regeneration may be carried out in a conventional reactor, such as a fixed-bed reactor, a fluidized-bed reactor or a transported-bed reactor.
The redox reaction (1) and the regeneration may therefore be carried out in a device with two stages, namely a reactor and a regenerator which operate simultaneously, and in which two batches of solid composition alternate periodically; the redox reaction (1) and the regeneration may also be carried out in the same reactor, by alternating the periods of reaction and regeneration.
Preferably, the redox reaction (1) and the regeneration are carried out in a reactor with a transported catalyst bed.
It is possible to use an operating mode with a single pass or with recycling.
According to a preferred embodiment, the propylene which is produced as a by-product, and/or the propane which has not reacted, are recycled (or returned) to the inlet of the reactor, that is to say they are re-introduced at the inlet of the reactor, mixed together with, or in parallel with, the initial mixture of propane, steam and, optionally, inert gas(es).